Aging our SNPs - again
This post should (hopefully) make a lot more sense...
A few of us have been doing some analysis to try to figure out how long ago our common ancestors may have lived. We have come up with much more recent estimates than previously determined.
SNP age calculations are always rough estimates. YFull uses 144.41 years per SNP to try to establish a formal baseline. The problem is that if you calculate the number of SNPs that some people have that are downstream of SNPs for which YFull has time estimates, and multiply that number by 144 years per SNP, this duration often does not align eve closely with YFull's time estimates. This simply proves that SNPs do not always occur at 144 years per SNP.
This is certainly the case with the people on our branch of the tree - the 10 Big-Y testers on the Z16357 branch have A LOT more SNPs than is typical. Based on generally accepted age estimates for older SNPs, we all average around 75 years per SNP. Our mutations occur nearly twice as fast on average as YFull's baseline - meaning that our common ancestors likely lived much more recently than YFull has estimated.
Daryl estimated that the time to our most recent common ancestor (TMRCA) that had the Z16357 SNP as being 2422 years ago. Below Z16357 is the large Z16343 block which he estimated ends at 900 years ago. Below it is Z17911 which he estimated at 706 years ago - much more recent than YFull's estimate of 1550 years ago. My own analysis closely aligned with Daryl's - though mine are bit older (I have Z17911 at around 800 years old).
We use a methodology that is based on assumed dates for very old SNP mutations, but that also considers the number of SNPs both upstream and downstream from a known SNP to establish a more reasonable estimation of time.
This approach gets more difficult as we get to the ends of the branches because we have fewer people that share those newer SNPs to analyze and average. But if we accept Z17911 as being ~800 years old, we can rough estimate the Bennett/Phillips ancestor as living ~550 years ago, for example.
I've updated the SNP chart at http://dna.smithplanet.com/snp with these refined estimates. As we get additional testers, especially those with known shared ancestors with other testers, then we'll be able to refine these estimates.